1. Field of the Invention
The present invention relates to a driving circuit for a simple matrix type display apparatus in which an input data signal is subjected to orthogonal transformation with an orthogonal function, and the transformed signal is subjected to reverse transformation for display on a side of a simple matrix type display apparatus.
2. Description of the Related Art
Conventionally, simple matrix type liquid crystal display apparatuses typified by Super Twisted Nematic (STN) liquid crystal display apparatuses are known.
This type of liquid crystal display apparatus has a structure in which a liquid crystal layer is interposed between two opposing glass substrates, and stripe-shaped scanning electrodes and data electrodes are disposed in a matrix so as to cross each other on the liquid crystal layer side of the glass substrate. In such a liquid crystal display apparatus, a voltage is applied to the scanning electrodes and the data electrodes, whereby liquid crystal at each crossed portion of the electrodes is supplied with an electric field. A display is performed by utilizing the rapid changes in optical characteristics of liquid crystal.
As described above, the simple matrix type liquid crystal display apparatus has a simple panel structure produced by a simple process, so that it can satisfy the requirements of screen enlargement at a relatively low cost.
An STN liquid crystal display apparatus is driven by time division driving (called a line sequential driving or Duty driving) which is described below.
In a simple matrix type liquid crystal display apparatus, a plurality of pixels are provided in one electrode, so that the pixels are driven with an applied voltage in the form of a time-divided pulse. In general, a group of scanning electrodes are scanned by line sequencing at a frame period of 20 ms or less. More specifically, a large selection pulse is applied to one scanning electrode only once per frame, and in synchronization with the pulse, a data signal corresponding to a display pattern is supplied from a data electrode to the pixels. This is repeated every horizontal synchronization period, thereby driving the pixels.
Liquid crystal which is driven as described above generally responds to an effective value of the driving voltage. That is, in a conventional STN liquid crystal display apparatus, the response speed of liquid crystal is relatively slow (i.e., about 300 ms), so that liquid crystal responds in accordance with an ON/OFF ratio of an effective voltage applied in line sequential driving. Thus, a practical optical contrast has been obtainable.
However, when a high response of liquid crystal which is capable of displaying moving images is realized by decreasing the viscosity of the liquid crystal and/or making a liquid crystal layer thinner in an STN liquid crystal panel, the liquid crystal molecules will have a faster response to a driving waveform. This deviates from a response to an effective value and consequently a so-called frame response phenomenon occurs.
The frame response phenomenon refers to a phenomenon where OFF transmittance is increased in a non-selected pixel (OFF display pixel), and actual transmittance is decreased in a selected pixel (ON display pixel) in spite of the fact that an optimum effective voltage is applied thereto. Thus, when conventional line sequential driving is applied to an STN liquid crystal panel having a high response, a display contrast markedly decreases.
In contrast, a multiple scanning line simultaneous selection method (called active driving as opposed to Duty driving) for simultaneously and selectively driving a plurality of scanning lines during one frame period has been described. According to the active driving method, a small scanning line selection pulse is applied to one scanning electrode a plurality of times during one frame period, and an accumulated response effect of liquid crystal is utilized, whereby the occurrence of the frame response phenomenon is suppressed in the high-response liquid crystal.
A specific driving circuit is shown in FIG. 11. As shown in this figure, an input image signal is subjected to orthogonal transformation in an orthogonal transformation circuit 101 which receives an orthogonal matrix from an orthogonal function 100. The transformed signal is supplied to a liquid crystal panel 104 by a data driver 102 from a data electrode side. The orthogonal matrix used for transformation is also supplied to the liquid crystal panel 104 as a scanning voltage pulse by a scanning driver 103 from a scanning electrode side. The transformed signal is then subjected to reverse transformation on the liquid crystal panel 104 side, whereby the input image signal is reproduced.
According to the active driving method, even when a selection pulse is simultaneously applied to a plurality of scanning electrodes, each pixel can be supplied with the same effective voltage as that in the conventional line sequential driving method. Thus, a normal display can be obtained.
The above-mentioned active driving method can be largely classified into two kinds, depending upon the method for selecting the scanning electrodes. One type of active driving method is an active addressing (AA) method (T. J. Scheffer, et al., SID '92, Digest, p. 228, Japanese Laid-open Publication No. 5-100642, and the like). According to the AA method, a WALSH function or the like is used as an orthogonal function, and a positive or negative voltage derived from the function is simultaneously applied to all the scanning electrodes. The other active driving method is a multiple line selection (MLS) method, typified by a sequence addressing method (T. N. Ruckmongathan et al., Japan Display 92, Digest, p. 65, Japanese Laid-open Publication No. 5-46127, and the like). According to the MLS method, one frame period is equally divided into a plurality of periods, and a plurality of different scanning lines in each period are simultaneously selected.
The orthogonal transformation operation of image data is an operation of a sum of products of a column direction data vector of a display image composed of selected data lines of elements and a column vector of an orthogonal function matrix. Data of a general image signal as used in TVs, displays for personal computers, and the like is conventionally scanned in a row direction; however, according to the active driving method, data is required to be arranged in a column direction. Thus, a data storage unit for temporarily storing data such as a frame memory is required for the purpose of rearranging a data signal.
The capacity of the data storage unit is affected by the structure of an orthogonal function matrix, i.e., the order of an operation during one frame period. According to the AA method and the dispersion type MLS method, because of the relationship of the order of the operation, a memory capacity for storing one frame of image data is required.
Furthermore, according to the AA method and the dispersion type MLS method, the same data signal is used a plurality of times during one frame period, whereby an orthogonal operation processing is completed. Therefore, when the content of data stored in a memory in one frame is changed, normal reverse transformation cannot be performed on a liquid crystal panel side.
Thus, in order to keep continuity of data between frames, another memory which allows a data signal of the next frame to be written while data is being read from a memory (i.e., during a data operation period of a frame) is required.
Hereinafter, the reason for the additional memory this will be described in detail.
In general, general-purpose memories such as a large-capacity dynamic random access memory (DRAM) have I/O in common, whereby the number (internally, bus width) of IC terminals is reduced. Therefore, I/O is appropriately switched in time sequence so that Read (out) and Write (in) processings are performed. Read (out) and Write (in) processings cannot be performed simultaneously. Thus, in the case where a double buffer processing is realized with a general-purpose memory such as an inexpensive DRAM, separate memories (i.e., a double buffer structure) for reading and writing are required.
Memory ICs, other than those of custom configurations, have a bit length (=bus width) and a word length (=address length) which determine the memory capacity of the ICs and are fixed in accordance with a certain rule (generally, exponetiation of 2). Thus, no matter how low the use efficiency of memories for reading or writing may be, independent memories for compensating for the required capacity for reading and writing are required.
Therefore, in a conventional display apparatus, as shown in FIG. 12, it is actually impossible to perform double buffer processing in which writing and reading are alternately performed by using 2 frames of memories A and B ("Study of a method for driving a high-speed response STN-LCD" (Kudo et al.), The Institute of Electric Communication Engineers of Japan, Study Report EID95-24, February, 1995). Instead, it is required for a large-capacity memory such as a frame memory to have a double buffer structure for orthogonal transformation of image data.
Thus, in a conventional driving circuit, irrespective of the degree of the use efficiency of memories, the total number of required memories (twice that required for a reading or writing processing) cannot be decreased, resulting in an increase in cost.